368 
GEOGEAPHICAL DISTEIBUTION, 
Chap. XI. 
have been due south and north. The Alpine plants/for 
example, of Scotland, as remarked by Mr. H. C. Watson, 
and those of the Pyrenees, as remarked by Eamond, are 
more especially allied to the plants of northern Scan¬ 
dinavia ; those of the United States to Labrador ; those 
of the mountains of Siberia to the arctic regions of that 
country. These views, grounded as they are on the 
perfectly well-ascertained occurrence of a former Glacial 
period, seem to me to explain in so satisfactory a 
manner the present distribution of the Alpine and 
Arctic productions of Europe and America., that when in 
other regions we find the same species on distant moun¬ 
tain-summits, we may almost conclude without other 
evidence, that a colder climate permitted their former 
migration across the low intervening tracts, since be¬ 
come too warm for their existence. 
If the climate, since the Glacial period, has ever been 
in any degree warmer than at present (as some geo¬ 
logists in the United States believe to have been the 
case, chiefiy from the distribution of the fossil Gnatho- 
don), then the arctic and temperate productions will at 
a very late period have marched a little further north, 
and subsequently have retreated to their present homes; 
but I have met with no satisfactory evidence with respect 
to this intercalated slightly warmer period, since the 
Glacial period. 
The arctic forms, during their long southern migra¬ 
tion and re-migration northward, will have been exposed 
to nearly the same climate, and, as is especially to be 
noticed, they will have kept in a body together; con¬ 
sequently their mutual relations will not have been 
much disturbed, and, in accordance with the principles 
inculcated in this volume, they will not have been liable 
to much modification. But with our Alpine productions, 
left isolated from the moment of the returning warmth, 
